Nonvolatile semiconductor memory device which performs improved erase operation

ABSTRACT

According to one embodiment, a nonvolatile semiconductor memory device includes a memory cell array and a control unit. The memory cell array includes a plurality of memory cells arranged in a matrix. The control unit erases data of the memory cells. The control unit interrupts the erase operation of the memory cells and holds an erase condition before the interrupt in accordance with a first command during the erase operation, and resumes the erase operation based on the held erase condition in accordance with a second command.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/180,541 filed Nov. 5, 2018, which is a divisional of U.S. applicationSer. No. 15/666,114 filed Aug. 1, 2017, which is a continuation of U.S.application Ser. No. 15/245,892 filed Aug. 24, 2016, which is acontinuation of U.S. application Ser. No. 14/677,484 filed Apr. 2, 2015,which is a continuation of U.S. application Ser. No. 14/022,944 filedSep. 10, 2013, which is a continuation of U.S. application Ser. No.13/052,158 filed Mar. 21, 2011, and is based upon and claims the benefitof priority from Japanese Patent Application No. 2010-182485, filed Aug.17, 2010; the entire contents of each of which are incorporated hereinby reference.

FIELD

Embodiments described herein relate generally to, for example, anelectrically rewritable nonvolatile semiconductor memory device and,more particularly, to a method of controlling the erase operationthereof.

BACKGROUND

One of electrically rewritable nonvolatile semiconductor memory devicesis a NAND flash memory. The NAND flash memory performs an eraseoperation for each block. In some systems, upon receiving a read orwrite operation request during the erase operation, the controller ofthe NAND flash memory issues an operation command for the read or writeafter the end of the erase operation. The erase operation usually takeslonger than the read or write operation. For this reason, much timeelapses from generation of the read or write operation request toexecution of the operation.

In the erase operation, the data of memory cells in a block are erasedfirst using the lowest initial voltage. After that, an erase verifyoperation is performed to confirm whether the memory cells are erased byverifying whether the threshold voltage of the memory cells reaches apredetermined threshold voltage. As a result, if the erase isinsufficient (the verify has failed), a step-up voltage is added to theinitial voltage to set a slightly higher erase voltage, and the eraseoperation is performed again. The series of operations is repetitivelyexecuted. The erase sequence ends when the erase verify has succeeded orreached a predetermined loop count.

Present memory cells may degrade at the time of the erase operation. Toprevent this, the erase operation is executed using a relatively lowinitial erase voltage and a low step-up voltage. Hence, there is atendency to increase the loop count and prolong the time required forthe erase operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a nonvolatile semiconductor memorydevice according to an embodiment;

FIGS. 2A and 2B are timing charts showing the operation of the firstembodiment;

FIG. 3 is a flowchart illustrating the operation of the firstembodiment;

FIG. 4 is a block diagram showing an example of a circuit that generatesa parameter to be used to generate an erase voltage according to thefirst embodiment;

FIG. 5 is a block diagram showing an example of a loop count circuitaccording to the first embodiment;

FIGS. 6A and 6B are timing charts showing the first modification of thefirst embodiment;

FIG. 7 is a flowchart illustrating the second modification of the firstembodiment;

FIGS. 8A and 8B are timing charts showing the operation of the secondembodiment;

FIG. 9 is a flowchart illustrating the operation of the secondembodiment; and

FIG. 10 is a block diagram showing a modification (third modification)of the second embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a nonvolatile semiconductormemory device includes a memory cell array and a control unit. Thememory cell array includes a plurality of memory cells arranged in amatrix. The control unit erases data of the memory cells. The controlunit interrupts the erase operation of the memory cells and holds anerase condition before the interrupt in accordance with a first commandduring the erase operation, and resumes the erase operation based on theheld erase condition in accordance with a second command.

Embodiments will now be described with reference to the accompanyingdrawings.

First Embodiment

This embodiment is to provide a nonvolatile semiconductor memory devicecapable of executing a read or write operation in a short time aftergeneration of a read or write operation request during an eraseoperation and also preventing occurrence of an overerase state afterresumption of the erase operation.

FIG. 1 shows the schematic arrangement of a NAND flash memory.

A memory cell array 1 includes a plurality of bit lines BL, a pluralityof word lines WL, a common source line SL, and a plurality of memorycells MC arranged in a matrix. Each memory cell MC is formed from, forexample, an EEPROM cell capable of electrically rewriting data. Aplurality of memory cells MC arranged in each column are connected inseries to form a NAND unit. Each NAND unit is connectable to acorresponding bit line and the source line via two selectiontransistors. Moreover, the memory cell array 1 includes a plurality ofblocks BLK1, BLK2 . . . BLKn. Each of the blocks BLK1, BLK2 . . . BLKncontains a plurality of the NAND units. Data of the memory cell array 1are erased by the block. A bit line control circuit 2 configured tocontrol the bit lines and a word line control circuit 6 are connected tothe memory cell array 1.

The bit line control circuit 2 reads data of the memory cells in thememory cell array 1 via the bit lines, detects the states of the memorycells in the memory cell array 1 via the bit lines, or applies a writecontrol voltage to the memory cells in the memory cell array 1 via thebit lines so as to write-access the memory cells. The bit line controlcircuit 2 includes, for example, two page buffers 2 a and 2 b serving ascache memories. The page buffers 2 a and 2 b hold data to be written inthe memory cells or data read from the memory cells for each page of thememory cell array 1. A column decoder 3 and a data input/output buffer 4are connected to the bit line control circuit 2.

A data input/output terminal 5 is connected to, for example, acontroller 10. The controller 10 outputs various kinds of commands CMD,addresses ADD, and data DT to control the operation of the NAND flashmemory. The data input/output terminal 5 receives the commands CMD,addresses ADD, and data DT output from the controller 10. Write datainput to the data input/output terminal 5 is supplied, via the datainput/output buffer 4, to a data storage circuit selected by the columndecoder 3, whereas commands and addresses are supplied to a control unit7.

The control unit 7 is connected to a control signal input terminal 8.The control signal input terminal 8 receives control signals such aschip enable (/CE), address latch enable (ALE), command latch enable(CLE), write enable (/WE), and read enable (/RE) output from thecontroller 10.

The control unit 7 controls the operation of the NAND flash memory basedon the various control signals supplied from the control signal inputterminal 8 and commands and addresses supplied from the datainput/output buffer 4.

A voltage generation circuit 9 is connected to the control unit 7. Thevoltage generation circuit 9 includes, for example, a pump circuit (notshown) and generates a program voltage VPGM serving as a high voltage inthe write mode, an intermediate voltage to be supplied to unselectedword lines in the write mode, a step-up voltage that slightly raises theprogram voltage VPGM, a verify voltage, a read voltage, an erasevoltage, a step-up voltage that slightly raises the erase voltage, andthe like. The voltages generated by the voltage generation circuit 9 aresupplied to the memory cell array 1, word line control circuit 6, bitline control circuit 2, column decoder 3, and the like.

The word line control circuit 6 is connected to the memory cell array 1.The word line control circuit 6 selects word lines in the memory cellarray 1 and applies a voltage necessary for read, write, or erase to theselected word lines.

The control unit 7 includes a plurality of registers 7 a, 7 b, and 7 c.When the erase operation is interrupted, registers 7 a and 7 b hold theerase voltage and the loop count of the erase operation which are beforethe interruption, respectively, as will be described later. Register 7 cholds the status at the time of, for example, interruption of the eraseoperation.

FIGS. 2A, 2B, and 3 illustrate the erase operation of the nonvolatilesemiconductor memory device having the above-described arrangement.FIGS. 2A and 2B show a continuous operation. The erase operation of thefirst embodiment will be explained with reference to FIGS. 2A, 2B, and3.

As shown in FIG. 2A, the erase operation starts upon receiving an eraseaddress input command, an address to select an erase block, and an erasecommand sequentially supplied from the controller 10. That is, thecontrol unit 7 starts the erase operation by setting a ready/busy signalin the busy state based on the erase command.

In the erase operation, for example, an initial erase voltage Vera1 isgenerated, and the loop count is cleared first by a setup operation, asshown in FIG. 3 (S11). Then, the control unit determines whether it isthe state after the erase has been interrupted (S12). This determinationis done based on the presence/absence of a precommand to be describedlater. In this case, since it is not the state after the erase has beeninterrupted, the initial erase voltage Vera1 is applied to the eraseblock for a predetermined time to execute the erase operation (S13).

After that, erase verify (Evfy) is performed (S14). The control unitthen determines based on the erase verify result whether the data hasbeen erased up to a predetermined erase level, or the loop count hasreached a predetermined value (S15). If the erase verify has failed (theerase is insufficient), and the loop count has not reached thepredetermined value, the erase voltage (erase pulse) is set up (S16).More specifically, a step-up voltage ΔVera is added to erase voltageVera1 to generate an erase voltage Vera2, as shown in FIG. 2A. Inaddition, the loop count is updated (S17). Erase voltage Vera2 isapplied to the selected block for a predetermined time to execute theerase operation again (S13). Note that the loop count may be updatedduring the erase operation. After that, erase verify is performed (S14).This operation is repeated until the erase verify succeeds (the erase issufficient), or the loop count reaches the predetermined value.

To interrupt the erase operation by the read operation, as shown in FIG.2A, the controller 10 supplies a reset command. The control unit 7forcibly interrupts the erase operation based on the reset command. Morespecifically, to interrupt during erase voltage application to theselected block, the erase voltage is immediately discharged to end theerase sequence. When the erase sequence has ended, the ready/busy signalis set in the ready state. After confirming the ready state of theready/busy signal, the controller 10 sequentially issues a read addressinput command, an address, and a read command. The control unit 7 setsthe ready/busy signal in the busy state based on the read command andexecutes the read operation to selected memory cells contained in ablock except for the erased block.

Upon receiving the reset command, the control unit 7 immediately endsthe erase sequence. The erase conditions before the reset are saved inregisters 7 a and 7 b. The erase conditions will be described later.

When the read operation has ended, and the data are serially output tothe outside, as shown in FIG. 2B, the interrupted erase operation can beresumed. However, if the normal erase operation command is issued, theerase operation starts from the initial erase voltage Vera1 and takesextra time to end, as described above. To prevent this, the operation isresumed from the erase voltage at the time of reset command input. Atcertain reset command input timing, however, if the erase operationstarts from the same erase voltage as that before the interrupt, theerase voltage is applied for more than the predetermined time. For thisreason, if the memory cells of the selected block have already beenerased to a predetermined threshold voltage, overerase occurs, resultingin degradation of the memory cells.

In the first embodiment, to distinguish the post-interrupt eraseoperation from the normal erase operation, a precommand is input beforethe normal erase operation command, as shown in FIG. 2B. Incorrespondence with the erase command sequence after the precommand, forexample, the erase operation is resumed from the erase voltage used inone loop before the interrupted loop, or erase verify is executed beforethe start of the erase operation to confirm whether the erase of theselected block has already succeeded.

In the first embodiment, erase verify is executed before application of,for example, an erase voltage Vera3, as shown in FIG. 2B. If the eraseverify has succeeded as a result, the erase operation ends. If the erasehas not succeeded, the erase voltage or the loop count is set up basedon the erase conditions at the time of interrupt held in registers 7 aand 7 b, and the erase operation is executed using them.

More specifically, upon receiving an erase address input command, anaddress, and an erase command after the precommand, the control unit 7determines that it is the post-interrupt erase operation (S12 to S14 ofFIG. 3). In this case, erase verify is executed before the eraseoperation (S14). If the erase verify has failed, the erase voltage isset up based on the erase conditions held in registers 7 a and 7 b(S16).

The erase condition held in register 7 a is, for example, a parameter todetermine the erase voltage. The erase condition held in register 7 bis, for example, the loop count. The erase voltage is determined by theparameter output from the control unit 7. Hence, when register 7 a holdsthe parameter value for determining the erase voltage at the time ofreset command input, it is possible to resume the erase operation bygenerating the same erase voltage as that at the time of reset commandinput based on the parameter value held in register 7 a.

The initial erase voltage for resumption can also be determined by theloop count held in register 7 b and the step-up voltage for the erase.Note that the circuit that generates and holds the parameter of theerase voltage and the circuit that generates and holds the loop countwill be described later.

The erase operation is thus resumed using the erase voltage that hasbeen set up.

Note that the reset command may be a reset command to be normally usedor a new reset command having the function of holding the state uponreset input in addition to the reset operation.

FIG. 4 shows a circuit that generates and holds the parameter to be usedto generate the erase voltage. This circuit includes, for example,selection circuits 21 and 22, an adder 23, and register 7 a. An initialvalue F_VERA of the parameter to generate the erase voltage is suppliedto one input terminal of selection circuit 21. A signal BINVERA outputfrom register 7 a is supplied to the other input terminal. One of theinput terminals of selection circuit 21 is selected by a signal LVE forloading the erase voltage or a signal PRC corresponding to theprecommand.

A parameter value F_ΔVERA to generate the step-up voltage is supplied toone input terminal of selection circuit 22. A parameter value VSS of theground voltage is supplied to the other input terminal. One of the inputterminals of selection circuit 22 is selected by a signal STU for setup.

The signals output from selection circuits 21 and 22 are supplied to theadder 23. The signal output from the adder 23 is supplied to register 7a.

According to the above-described arrangement, in the normal eraseoperation, the initial value F_VERA of the erase voltage is selectedbased on, for example, signal LVE. Selection circuit 22 selects, forexample, VSS based on the disabled signal STU. The signals output fromselection circuits 21 and 22 are supplied to the adder 23 and added.Signal F_VERA+VSS output from the adder 23 is the initial value F_VERAin fact, which is supplied to register 7 a. The initial value F_VERAheld in register 7 a is supplied to the voltage generation circuit 9 asthe parameter BINVERA to be used to generate the erase voltage.

If the erase verify after the first erase voltage has been supplied tothe selected block has failed, selection circuit 21 selects the outputsignal from register 7 a as signal LVE is disabled. Selection circuit 22selects the step-up voltage F_ΔVERA as signal STU is enabled. For thisreason, the adder 23 outputs F_VERA+F_ΔVERA which is held in register 7a. The second erase voltage Vera2 is generated based on this parameter.This operation is repeated until the verify succeeds.

On the other hand, to resume the erase operation after interrupt,selection circuit 21 selects the output signal BINVERA from register 7 abased on signal PRC corresponding to the precommand. Selection circuit22 selects, for example, VSS as signal STU is disabled. For this reason,the adder 23 actually outputs the initial value BINVERA.

That is, for example, if the erase operation is interrupted duringapplication of erase voltage Vera3, as shown in FIG. 2A, erase voltageVera3 can be generated at the time of resumption, as shown in FIG. 2B.

FIG. 5 illustrates a circuit that counts and holds the loop count. Thiscircuit includes a counter 31, a reset circuit 32, and register 7 b. Thecounter 31 updates the loop count based on an increment enable signal.The reset circuit 32 resets the count value of the counter 31 based on areset signal and supplies the value to register 7 b. Upon receiving theprecommand, the reset circuit 32 supplies the count value of the counter31 to register 7 b without resetting.

Hence, in the erase operation, if the verify has failed, the counter 31increments the loop count by one based on the increment enable signal.This count value is held in register 7 b via the reset circuit 32. Thisoperation is repeated until the verify succeeds, or the loop countreaches a predetermined value.

On the other hand, to resume the erase operation, count value reset bythe reset circuit 32 is prohibited based on the precommand. Hence, theloop count is updated by setting the count value held in register 7 bbefore the interrupt as the initial value.

According to the first embodiment, if the reset command is issued duringthe erase operation, the erase operation is interrupted, and theparameter value of the erase voltage at the time of interrupt and theloop count of the erase operation are saved in registers 7 a and 7 b.When resuming the erase operation, the erase voltage can be generatedbased on the parameter value of the erase voltage held in register 7 aor the loop count of the erase operation. It is therefore possible toshorten the time needed for the erase operation.

Additionally, when resuming the erase operation, a precommand is issuedprior to the erase operation command. The precommand allows todistinguish the post-interrupt erase operation. In the post-interrupterase operation, erase verify is performed before the erase operation todetermine whether the erase has succeeded. It is therefore possible toprevent overerase of the memory cells.

First Modification

FIGS. 6A and 6B show the first modification of the first embodiment.

In the first embodiment, after serially outputting the data read byinterrupting the erase operation, the erase operation is resumed usingthe precommand and the normal erase operation command. In the firstmodification, however, a cache erase command is used in place of theerase command, as shown in FIG. 6B, to resume the erase operation beforeserial output of the data read by interrupting the erase operation. Thecache erase command can load the next write data to the cache during theerase operation. This allows the erase operation and data load tooverlap.

In the first modification, serial output of the data read byinterrupting the erase operation is made to overlap the erase operationusing the cache erase command. That is, serial output of data andresumption of the erase operation can be executed simultaneously, asshown in FIG. 6B.

However, when the normal cache erase command is issued, the cache, thatis, the page buffer is reset, and load of the next write data isexecuted during dummy busy shown in FIG. 6B. For this reason, the pagebuffer that holds the read data is reset during the cache erasesequence, and the read data is lost. To prevent this, the command needsto be changed to prohibit the page buffer reset operation in the firstmodification. That is, whether or not to reset the page buffer in thecache erase sequence is determined based on the presence/absence of theprecommand. More specifically, if the precommand is detected, the pagebuffer reset operation in the cache erase sequence is avoided.

According to the first modification, serial output of data read byinterrupting the erase operation can overlap the erase operation. Thisallows to shorten the total operation time including the erase operationand the data output operation.

Second Modification

FIG. 7 illustrates the second modification of the first embodiment.

In the erase operation, preprogram for performing a write operation inall cells of the selected block before erase voltage application or softprogram for slightly raising the threshold voltage after the end oferase (after erase verify has succeeded) may be executed. Theabove-described reset command may be input during the preprogram or softprogram. In the second modification, control is performed, for example,in the following way to cope with this situation. Note that the softprogram may write-access only memory cells connected to dummy word linesbut not the remaining memory cells.

Steps S21 to S29 of FIG. 7 represent the erase sequence including thepreprogram. This erase sequence is almost the same as in FIG. 3 exceptsteps S22 and S23 of the preprogram.

After setup (S21), the control unit determines the preceding status inwhich reset has occurred (S22). That is, the control unit determineswhich is the preceding status, preprogram, erase, erase verify, or softprogram. This determination is done based on, for example, status dataheld in the status register 7 c provided in the control unit 7. In theinitial state, the status data held in the status register 7 c indicatesnone of the above statuses. For this reason, preprogram is executed(S23). After that, the erase operation (S24) and the erase verifyoperation (S25) are executed, and the verify result is determined (S26).If the verify result indicates a failure, and the loop count has notreached the predetermined value, the erase voltage is set up (S27), theloop count is updated (S28), and the erase operation is executed again.

Steps S31 to S38 represent the soft program sequence after the eraseverify has succeeded. In the soft program sequence, write is slightlyexecuted to narrow the threshold voltage distribution in the erasestate. That is, in soft program S31, the slight write operation isexecuted by setting a program voltage. After that, verify is executedbased on a predetermined verify voltage. The soft program sequenceincludes two verify steps. The first verify is intelligent soft programverify (S32 and S33). The second verify is soft program verify (S37 andS38). The intelligent soft program verify and the soft program verifyuse different verify voltages. More specifically, the verify voltage ofintelligent soft program verify is set to be lower than that of softprogram verify. Hence, the intelligent soft program verify is firstexecuted to verify whether the threshold voltage of the memory cells iswritten back to a distribution of certain degree. The soft programverify is then performed to verify whether the write-back has not beendone excessively.

That is, after the soft program (S31), the intelligent soft programverify is performed (S32 and S33). If the write-back is not sufficient,the soft program voltage is stepped up (S34), the loop count is updated(S35), and the soft program is executed again (S31). This operation isrepeated until the verify succeeds, or the loop count reaches thepredetermined value.

After that, the control unit determines whether the intelligent softprogram verify has failed (S36). That is, it is determined whether theverify has not succeeded, and the loop count has reached thepredetermined value. If the intelligent soft program verify hassucceeded, soft program verify is executed to determine whether thewrite-back has not been done excessively (S37 and S38). If write-backhas been done excessively, the control returns to step S21 to executethe erase operation again.

If the erase verify or the intelligent soft program verify has failed,the erase processing ends, and, for example, error processing isexecuted.

On the other hand, to resume the interrupted erase, setup is performed(S21), and after that, the state at the time of erase interrupt isdetermined based on the status data held in the status register 7 c(S22). If the determination result reveals that reset has been done uponreceiving the reset command during the preprogram (S23), for example,the control advances from preceding status determination step S22 toerase step S24 to resume the erase sequence from the beginning, that is,application of the first erase voltage. Alternatively, the preprogram isexecuted from the beginning or resumed from the interrupt timing.

If the reset command has been input during application of the firsterase voltage, the erase sequence may be executed again from applicationof the first erase voltage or resumed from the erase verify. If thereset command has been input during the soft program, the erase sequenceis executed from the first soft program.

According to the second modification, even if the reset command isissued during the erase sequence including preprogram or soft program,and the erase sequence is interrupted, it is possible to perform theerase operation in a short time after resumption and prevent overerase.

In addition, the first and second modifications allow to insert theread/write operation during the erase operation, using relatively simplearrangements without any large change in the existing circuits.

Second Embodiment

FIGS. 8A, 8B, and 9 show the second embodiment. The same step numbers asin FIG. 7 denote the same processes in FIG. 9.

In the first embodiment, it is necessary to input the reset commandduring the erase operation to interrupt the erase operation and theninput the precommand and the normal erase command to resume the eraseoperation using the same erase voltage as that before the interrupt.

However, the second embodiment makes it possible to obviate the need ofcommand input for resuming the erase operation and interrupt one erasesequence by the read/write operation.

More specifically, as shown in FIG. 8A, upon receiving an erase addressinput command, an address, and a command called, for example, a suspenderase command from a controller 10 in the erase operation, a ready/busysignal is set in a dummy busy state. In addition, an internal ready/busysignal is set in a busy state to start the erase operation. The eraseoperation is the same as in the first embodiment. The internalready/busy signal is a response signal inside a control unit 7.

When the ready/busy signal returns from the dummy busy state to theready state, a command for interrupting the erase by the read or writeoperation can be issued. In this state, upon receiving, for example, aread address input command, an address, and a read command sequentiallysupplied from the controller 10, the control unit sets the ready/busysignal in the busy state.

Based on the read command, the control unit 7 issues an interruptrequest at a timing to be described later in synchronism with aninternal clock signal. The erase operation is thus interrupted, and theread operation starts immediately.

The interrupt request is issued to interrupt the erase operation at atiming after preprogram (S23), after erase voltage application (S24),after erase verify (S25), or after soft program (S31).

Note that to shorten the time from the interrupt request to the start ofthe read operation as much as possible, forcible termination may occurduring preprogram, during erase voltage application, or during softprogram. The status when the interrupt request has occurred is held in,for example, a register 7 c.

Forcible termination during erase voltage application is explained thebelow as an example. When the interrupt request is issued while theerase voltage is supplied, the erase voltage may be forcibly dischargedwithout waiting for a predetermined time for supplied erase voltage.When the interrupt has occurred after forcible discharge, the controlunit 7 first executes an erase verify operation after resumption of theerase operation. If the verify result indicates a success, the eraseoperation ends without applying the erase voltage to the memory cells.If the verify result indicates a failure, the erase may be started usingthe same voltage as that before the interrupt.

When the read operation has ended, and the interrupt operation has thusended, the ready/busy signal returns to the ready state, as shown inFIG. 8B, so that the data read by the read operation can be output. Atthis time, The control unit 7 resumes the interrupted operation based onthe status data held in register 7 c.

More specifically, at the time of resumption, the operation is executedfrom the erase verify if the interrupt has occurred at a timing afterthe erase pulse application, as shown in FIG. 9. If the interrupt hasoccurred at a timing after the erase verify, the erase voltage is setup, the loop count is updated, and the erase voltage application starts.

If the interrupt has occurred during preprogram, one of the followingoperations is executed. (1) The preprogram is resumed from thebeginning. (2) The operation is resumed from application of the firsterase voltage. (3) The preprogram is resumed from the interruptedportion. (4) The interrupt operation starts after the end of preprogram.

If the interrupt has occurred during application of the first erasevoltage, the operation may be resumed again from application of thefirst erase voltage or from the erase verify. If the interrupt hasoccurred during soft program, the operation is resumed from the firstsoft program. The resumed operation can be done simultaneously with theoperation of outputting the read data held in the page buffer to theoutside.

If the forcible termination has occurred during erase voltageapplication, the operation starts from the erase verify to preventovererase of the memory cells, as in the first embodiment.

Alternatively, the erase verify result may be held in one page bufferduring the erase operation. Then, the read operation may be executed tointerrupt the erase operation, and data read by the read operation maybe held in the other page buffer. After resumption of the eraseoperation, the data held in the other page buffer may be output to theoutside.

A case in which the erase operation is interrupted by the read operationhas been described above. However, the embodiment is not limited tothis. The same operation as described above can be executed even whenthe erase operation is interrupted by the write operation.

In this case, the erase verify result may be held in one page bufferduring the erase operation, and write data may be held in the other pagebuffer during the erase operation. The write operation may be executedto interrupt the erase operation using the data held in the other pagebuffer.

According to the second embodiment, upon receiving the erase command,the erase operation is started by setting the ready/busy signal in thedummy busy state and the internal ready/busy signal in the busy state.After that, the ready/busy signal is returned from the dummy busy stateto the ready state to enable read or write command input. When the reador write command is input, the erase operation is interrupted. When theinterrupt operation has ended, the erase operation is resumed. For thisreason, the command to resume the erase operation need not be input. Inaddition, if the interrupt is the read operation, the operation ofoutputting data to the outside can overlap the erase operation. If theinterrupt is the write operation, data input to the page buffer canoverlap the erase operation. It is therefore possible to shorten theoperation time by simultaneously performing the erase operation and thedata read or data input operation.

FIG. 10 illustrates a modification (to be referred to as thirdmodification) of the second embodiment. For example, when the writeoperation (program) interrupts the erase operation, it is sometimesdesirable to know whether the program verify has succeeded or failed.The result of program verify representing whether it has succeeded orfailed can be known by causing the controller 10 to issue a statuscommand.

In the second embodiment, however, the erase operation and the programoperation can simultaneously be executed. Hence, if the status commandis simply issued, it is difficult to distinguish between the status ofthe erase operation and that of the program operation.

For this purpose, as shown in FIG. 10, a status register 41 for normaloperation and a status register 42 for suspend erase which holds thestatus in case of use of a suspend erase command are connected to thecontrol unit 7. The status register 41 holds a status at the time ofprogram verify. The status register 42 for suspend erase holds a statusrepresenting an erase success or failure when the erase has beenexecuted using the suspend erase command. That is, the status register42 for suspend erase holds a status representing whether all of theerase verify, intelligent soft program verify, and soft program verifyhave succeeded or failed.

The output terminals of the status register 41 and the status register42 for suspend erase are connected to a selection circuit 43. Selectioncircuit 43 can select one of the status register 41 and the statusregister 42 for suspend erase based on a status command for normaloperation and a status command for erase interrupt. When the controller10 issues the status command for normal operation, the status of programverify held in the status register 41 is selected and sent to thecontroller 10 via a data input/output buffer 4 as status data. When thecontroller 10 issues the status command for erase interrupt, the statusat the time of erase execution held in the status register 42 forsuspend erase is selected and sent to the controller 10 via the datainput/output buffer 4 as status data.

According to the third modification, the status upon executing erase andthe status of program verify can reliably be extracted. It is thereforepossible to correctly recognize the operating state of the nonvolatilesemiconductor memory device.

Note that referring to FIGS. 7 and 9, the erase sequence includesdifferent operations such as preprogram and soft program in addition tothe erase operation. The preprogram and soft program may be separatedfrom the erase sequence and independently performed using commands.

In addition, both the first and second embodiments are applicable to theerase interrupt sequence.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A nonvolatile semiconductor memory devicecomprising: a bit line; a source line; a memory cell array including aplurality of blocks, each block including a plurality of memory cellsconnected in series between the bit line and the source line; and acontrol unit configured to: perform an erase operation upon receipt ofan erase command, the erase operation including a plurality of erasevoltage apply steps and a plurality of erase verify steps; suspend theerase operation upon receipt of a suspend command, the erase operationbeing suspended at completion of one of the erase voltage apply steps;perform a first operation upon receipt of a first command, an operationtime of the erase operation being longer than an operation time of thefirst operation; and resume the erase operation upon receipt of a resumecommand, the erase operation being resumed at start of one of the eraseverify steps.
 2. The device according to claim 1, further comprising afirst terminal configured to receive the erase command, the suspendcommand, the first command, and the resume command from an exterior ofthe device.
 3. The device according to claim 2, wherein the firstoperation includes a read operation, and the first command includes aread command.
 4. The device according to claim 1, wherein the controlunit includes a status register configured to hold status information,and configured to output the status information to an exterior of thedevice upon receipt of a status command.
 5. The device according toclaim 4, wherein the status information output from the status registerindicates a success or failure of the erase operation at a time ofsuspend of the erase operation.
 6. The device according to claim 1,further comprising a second terminal configured to output a ready/busysignal.
 7. The device according to claim 6, wherein an indication of theready/busy signal output from the second terminal changes from a readystate into a busy state upon receipt of the erase command, and changesfrom the busy state into the ready state upon receipt of the suspendcommand after the erase command.
 8. The device according to claim 7,wherein the indication of the ready/busy signal output from the secondterminal changes from the ready state into the busy state upon receiptof the first command after the suspend command, and changes from thebusy state into the ready state upon completion of the first operation.9. The device according to claim 8, wherein the indication of theready/busy signal output from the second terminal changes from the readystate into the busy state upon receipt of the resume command, andchanges from the busy state into the ready state upon completion of theresumed erase operation.
 10. The device according to claim 7, whereinthe control unit is configured not to perform an erase verify stepsubsequent to receipt of the suspend command.
 11. The device accordingto claim 7, wherein the control unit is configured not to perform anerase voltage apply step subsequent to receipt of the resume commandbefore the one of the erase verify steps has been performed.
 12. Thedevice according to claim 3, wherein the control unit is configured tooutput read data before receiving the resume command.
 13. The deviceaccording to claim 3, wherein the control unit is configured to performthe erase operation on a selected block upon receipt of the erasecommand, acid perform the read operation on another block upon receiptof the read command after the suspend command.
 14. A memory systemcomprising: a nonvolatile semiconductor memory device; and a controller,wherein the controller is configured to issue an erase command, asuspend command, a first command, and a resume command, the nonvolatilesemiconductor memory device includes: a hit line; a source line; and amemory cell array including a plurality of blocks, each block includinga plurality of memory cells connected in series between the bit line andthe source line, and the nonvolatile semiconductor memory device isconfigured to: perform an erase operation upon receipt of the erasecommand from the controller, the erase operation including a pluralityof erase voltage apply steps and a plurality of erase verify steps;suspend the erase operation upon receipt of the suspend command from thecontroller, the erase operation being suspended at completion of one ofthe erase voltage apply steps; perform a first operation upon receipt ofthe first command from the controller, an operation time of the eraseoperation being longer than an operation time of the first operation;and resume the erase operation upon receipt of the resume command fromthe controller, the erase operation being resumed at start of one of theerase verify steps.
 15. The system according to claim 14, wherein thenonvolatile memory device further includes a first terminal configuredto receive the erase command, the suspend command, the first command,and the resume command from the controller.
 16. The system according toclaim 15, wherein the first operation includes a read operation, and thefirst command includes a read command.
 17. The system according to claim14, wherein the nonvolatile semiconductor memory device further includesa status register configured to hold status information, and configuredto output the status information to the controller upon receipt of astatus command.
 18. The system according to claim 17, wherein the statusinformation output from the status register indicates a success orfailure of the erase operation at a time of suspend of the eraseoperation.
 19. The system according to claim 14, wherein the nonvolatilesemiconductor memory device further includes a second terminalconfigured to output a ready/busy signal.
 20. The system according toclaim 19, wherein an indication of the ready/busy signal output from thesecond terminal of the nonvolatile semiconductor memory device changesfrom a ready state into a busy state upon receipt of the erase commandfrom the controller, and changes from the busy state into the readystate upon receipt of the suspend command from the controller after theerase command.
 21. The system according to claim 20, wherein theindication of the ready/busy signal output from the second terminal ofthe nonvolatile semiconductor memory device changes from the ready stateinto the busy state upon receipt of the first command from thecontroller after the suspend command, and changes from the busy stateinto the ready state upon completion of the first operation from thecontroller.
 22. The system according to claim 21, wherein the indicationof the ready/busy signal output from the second terminal of thenonvolatile semiconductor memory device changes from the ready stateinto the busy state upon receipt of the resume command from thecontroller, and changes from the busy state into the ready state uponcompletion of the resumed erase operation.
 23. The system according toclaim 20, wherein the nonvolatile semiconductor memory device isconfigured not to perform an erase verify step subsequent to receipt ofthe suspend command from the controller.
 24. The system according toclaim 20, wherein the nonvolatile semiconductor memory device isconfigured not to perform an erase voltage apply step subsequent toreceipt of the resume command from the controller before the one of theerase verify steps has been performed.
 25. The system according to claim16, wherein the nonvolatile semiconductor memory device is configured tooutput read data before receiving the resume command from thecontroller.
 26. The system according to claim 16, wherein thenonvolatile semiconductor memory device is configured to perform theerase operation on a selected block upon receipt of the erase commandfrom the controller, and perform the read operation on another blockupon receipt of the read command after the suspend command from thecontroller.
 27. The device according to claim 1, wherein upon receipt ofthe resume command, after the one of the erase verify steps isperformed, a next one of the erase voltage apply steps is started, inthe one of the erase voltage apply steps, a first erase voltage isapplied, and in the next one of the erase voltage apply steps, a seconderase voltage higher than the first erase voltage is applied.
 28. Thesystem according to claim 14, wherein upon receipt of the resumecommand, after the one of the erase verify steps is performed, a nextone of the erase voltage apply steps is started, in the one of the erasevoltage apply steps, a first erase voltage is applied, and in the nextone of the erase voltage apply steps, a second erase voltage higher thanthe first erase voltage is applied.